Selective Gas Uptake and Rotational Dynamics in a (3,24)-Connected Metal-Organic Framework Material.

The desolvated (3,24)-connected metal-organic framework (MOF) material, MFM-160a, [Cu(L)(HO)] [HL = 1,3,5-triazine-2,4,6-tris(aminophenyl-4-isophthalic acid)], exhibits excellent high-pressure uptake of CO (110 wt% at 20 bar, 298 K) and highly selective separation of C hydrocarbons from CH at 1 bar pressure. Henry's law selectivities of 79:1 for CH:CH and 70:1 for CH:CH at 298 K are observed, consistent with ideal adsorption solution theory (IAST) predictions. Significantly, MFM-160a shows a selectivity of 16:1 for CH:CO. Solid-state H NMR spectroscopic studies on partially deuterated MFM-160- confirm an ultra-low barrier (∼2 kJ mol) to rotation of the phenyl group in the activated MOF and a rotation rate 5 orders of magnitude slower than usually observed for solid-state materials (1.4 × 10 Hz cf. 10-10 Hz). Upon introduction of CO or CH into desolvated MFM-160a, this rate of rotation was found to increase with increasing gas pressure, a phenomenon attributed to the weakening of an intramolecular hydrogen bond in the triazine-containing linker upon gas binding. DFT calculations of binding energies and interactions of CO and CH around the triazine core are entirely consistent with the H NMR spectroscopic observations.